Medicine balls have existed for literally thousands of years, with the earliest documented use by Persian wrestlers who trained with animal bladders or skins filled with sand. They are primarily used to create explosive power in athletes (e.g., by developing muscles necessary to propel a medicine ball vertically upward as fast as possible) as well as to increase strength in the core of the body (e.g., by performing sit-ups with a medicine ball).
To progressively create more explosive power or increase core strength, an athlete needs medicine balls of different weights. But purchasing multiple medicine balls is expensive and takes up storage space. Therefore, a need exists for a medicine ball to which weight can be added, in order to reduce cost and the need for multiple pieces of equipment.
Currently, some medicine balls to which weight may be added do exist. However, in these currently existing medicine balls, when additional “exercise” weights are added the ball may be “unbalanced” meaning that the center of mass (or center of gravity) of the ball is not at the geometric center of the ball. A medicine ball patent over one hundred years old (U.S. Pat. No. 777,478 to Minor) discloses the addition of weights. However, the weights are added at the periphery of the medicine ball in one of two cavities in such a way as to cause the center of mass to not be located at the center of the ball. Offsetting the center of mass causes the ball to wobble as it rotates thus making it difficult to hold, throw, and/or catch. Other patents, including U.S. Pat. No. 8,454,483 to Bradley et al., and U.S. Pat. No. 6,387,022 to Smith also disclose variations on the adjustable weight medicine ball. But these patents also neglect to take into consideration the effect of weight addition on the center of mass.
The unbalance causes the ball to wobble when it is thrown through the air, making it difficult to throw and/or catch, thus increasing the chance of injury to the person catching the ball. The unbalance can also make the ball difficult to hold onto when it is moved during strengthening exercises. Earlier inventions did not address the issue of balancing the ball in all coordinate directions when additional weights are added. Therefore, a need exists for a variable weight medicine ball that remains balanced and creates a minimum amount of wobble when different weights are added.